Method of centrifugal casting



Patented May 20,1941

METIlOD F CENTRIFUGAL CASTING Walter G. Donald, Cambridge, and James L. Martin, Lowell, Mass.

No Drawing. Application December 19,1938,

Serial No. 246,647

4 Claims. (of 22-65) (Granted under the act of March. 3, 1883; as amended April 30, 1928; 370 O. G. 757) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to r us of any royalty thereon. v

This inventionrelates to a method of centrifugally casting iron in chill'molds and more particularly to controlling the gradient from white iron to gray iron in cylindrical or similar cast iron castings of the class'having an outside diameter on the order of four to six times the inside. diameter or equivalent wall thickness, which class will hereinafter be referred to generally as thick-walled cylinders or castings.

Heretofore, where the need has arisen for thick-walledcylinders or castings capable of at once withstanding high pressures and offering a wear and corrosion resistant interior surface, it has been necessary to either compromise -on a single material notcomplying with the full requirements of the need, or to use concentrically bonded materials wherein trouble has been experienced ,in obtaining a satisfactory bond.

This necessity has existed because of the dimculty in avoiding segregation during freezing of the large volume of metal used in thick-walled cylinders or similar castings, which causes the actual composition of the metal to widely vary in different portions of the casting. If the molten metal or heat is poured into an ordinary stationary chill mold of a size to give a thick-walled casting of any appreciable length, both longitudinal and transverse segregation results which is either exceedingly difllcult or impossible to control within economically feasible limits, this being especially true of longitudinal segregation.

On the other hand longitudinal segregation is avoided in the usual centrifugal cast iron castings as made in the art at present, but such castings are so relatively thin-walled that little if any gradient of composition of any type results in the casting, consequently, the fabrication of a thick-walled cylindrical or similar member of two or more such castings concentrically or contiguously arranged does not result in a structure having a uniform gradient from the outer surface of the casting to the bore or inner side thereof.

It is an object of this invention to set forth a method for casting thick-walled cast iron cylindrical or similar castings which incorporates steps heretofore employed for chilling the sur faces of similar castings of short length; such as railway can wheels and mill rolls, with present known centr 1 ugal casting methods, in order to eliminate longitudinal segregation, and thereby render available an economical method of producing castings of this class in substantial lengths.

It is a further object of this invention to con- V trol the radial segregation of carbon in thickwalled centrifugal castings by controlling the chill ratio of the mold relative to the casting to effect a predetermined gradient from white iron to gray iron.

While the essential objects and-steps of this invention have been outlined above the same consists particularly in the steps to be hereinafter set forth, it being understood, however,

that the description is for the tration and not limitation.

If a thick-walled cast iron cylinder or similar casting be cast centrifugally in a horizontally disposed cylindrical or other appropriate chill mold spun about its longitudinal axis under conditions insuring a continuous uniform flow of molten metal throughout the full length of the mold, longitudinal segregation will be eliminated due to the radial action of centrifugal force upon the flowing. metal. At the same-time however, due to the solidification process and/or centrifugal force radial segregation of the carbon and other elements or impurities will be effected, and it is the control of this radial segrepurpose of illusgation of carbon, through a control of the chill ratio between the mold and casting that we have provided for regulating the gradient from white iron to gray iron in thick-walled cylindrical or similarxcastings.

We have found that since segregation is not due to centrifugal force alone; but, is also efiected by the solidification process through which the molten metal passes in freezing, that there are several factors in any given set of conditions .affecting the solidification process that must be correlated to each other and kept constant in order to effect a control of the gradient from white iron to gray iron solely through a control of the. chill ratio between the mold and by arranging the other factors afiectingthis gradient as constants. For convenience of consideration the volumetric relation of the mold and casting is expressed as a ratio between the cross-sectional area of the mold and casting and such ratio is referred to as the chill ratio.

Although any suitable type of chill moldmay be utilized in this process, we prefer to use chill molds made of gray iron, as they lend themselves to more convenient adaptation in a centrifugal process.

In chill molds of any type the heat from the casting is of course absorbed by the mold through conduction and then transferred to the surrounding atmosphere by radiation. The heat absorbed by the mold will be transferred more rapidly, through conduction, from the inner surface of the mold to the outer surface than it will be, by radiation, from the latter surface to the surrounding atmosphere; therefore, the larger the volume of the mold, or the thicker the wall of the mold the greater will be the chilling action on the casting. It then follows that if the volumetric relation or chill ratio of the mold to a given casting be changed that the chilling action will likewise be changed. By increasing the chill ratio for a given size casting the metal will solidify more quickly, which will not allow physical segregation of the carbon under the influence of centrifugal force to the same extent that will obtain under conditions of slower solidification. As the gradient from white iron to gray iron depends upon the separation of graphitic carbon and growth of graphitic plates it should be apparent that a control of the chill ratio to control the length of time the metal is under the influence of centrifugal force in the molten state will also control the depth of the outer white iron zone if the other factors affecting solidification and segregation be kept constant.

As examples of the control that may be attained over the depth of the white iron ring or zone, through variation in the chill ratio, when the remaining factors above enumerated are kept constant the following is given for a constant normal composition.

Radial i Pour tem a White iron a non Rx. No. 3225383 in degree zone or or in inches Fahrenheit ring in zone in inches inches It will be noted that the depth of the white determining the depth of white iron zone formed therein through regulation of the chill ratio of the mold to the casting, the steps which comprise selecting an iron having a composition adapted to be solidified into a tubular casting of white iron having an outside diameter between the limits of 4 to 6 times as great as the inside diameter thereof, selecting a chill mold having a chill ratio with respect to the casting between the limits of T and in accordance with the depth of white iron zone predetermined, rotating the mold about its axis and casing said selected iron into the mold at a constant temperature.

. 2. In the method of forming cast iron cylindrical tubular articles of the class having an outside diameter of 4 to 6 times the inside diameter, the steps which comprise providing a rotatable chill mold having its cross-sectional area proportioned with respect to the cross-sectional area of a tubular article of said diameters in a ratio between the limits of 6.511 and 1:1 in accordance with a predetermined depth of white iron zone to be formed in the article between the limits of 0% and selecting a cast iron adapted to be solidified between the limits of all white iron and all gray iron, rotating the mold and pouring said iron in molten form into the mold at a constant temperature.

3. In the method of forming cast iron cylindrical tubular articles of the class having an outside diameter of 4 to 6 times the inside diamter, the steps which comprise selecting a cast iron adapted to be solidified from the molten state into a tubular article of said diameters as all white'iron or all gray iron, providing a gray iron chill mold having its cross-sectional area proportioned with respect to the cross-sectional area of a tubular article in accordance with a predetermined depth of white iron zone to be formed in said article and between the limits of 6.521 and 1:1, rotating said mold, and pouring the iron in molten form into the mold at a constant temperature.

4. In the method of forming cast iron cylindrical tubular members of the class having a radial thickness of 2 inches the steps which comprise, selecting an iron adapted to be solidified from the molten state into a tubular article of said radial thickness between the limits of and including all white iron or all gray iron, providing a gray iron chill mold having its cross-sectional area proportioned with respect to the cross-sectional area of a tubular article ofradial thickness of 2 inches in a ratio between the limits of 6.521 and 1:1 in accordance with a predetermined depth of white iron zone between the limits of 2 inches and 1.3 inches,.rotating said mold, and pouring the iron in molten form into the mold at a temperature of 2950 F.

WALTER G. DONALD. JAMES L. MARTIN. 

